It is known, e.g., from U.S. Pat. No. 3,505,618 of McKee, that a radio frequency band-pass filter may be formed from a generally right parallelepiped body of dielectric material having top, bottom, side, and end surfaces. Holes are formed in the body extending from the top surface toward the bottom surface. A conductive material is coated over the most of the outer surfaces, except perhaps the top surface, and extends into the holes in order to form transmission line resonators. The conductive material in the holes is electrically connected to the conductive material on the bottom surface of the dielectric block. However, at the top surface the conductive material of the holes is not connected to the conductive outer coating. As a result, the resonators have a short circuit end toward the bottom surface of the dielectric block and an open circuit end at the top surface.
Means are provided for coupling a signal into and out of the endmost holes, e.g., by means of plug-type electrodes fitted into the open circuit ends of these holes. As an alternative to coupling into the dielectric block by means of plug-type electrodes, it is known to couple capacitively to the open circuit end of the resonator by means of conductive strips or electrodes formed on the top, end or side surfaces of the dielectric block. This type of coupling is described in U.S. Pat. Nos. 4,431,977 of Sokola et al., No. 4,692,726 of Green et al. and No. 4,716,391 of Moutrie et al. Conductive electrode pads that are isolated from the other conductive material, are coated on one of these surfaces of the dielectric material adjacent one of the resonator holes. An input or output lead is also connected to the pad. By locating the pad toward the open circuit end of the resonator, the signal on an input lead affects the electric field surrounding the open circuit end of the resonator, and capacitively induces a signal into the dielectric block. Alternatively, the pad at the output intercepts the electric field and picks up a signal from the block which it induces in the output lead.
In one embodiment disclosed in the Sokola et at. patent, an electrode is placed on an end surface near the short circuit end of the resonator. This electrode is coupled to the conductive material at the bottom of the block and an input lead is coupled to the electrode. As a result, the signal on the input lead forms a current that affects the magnetic field around the short circuit end of the resonator, and inductively induces a signal into the dielectric block. A similar output electrode and lead inductively pick up a signal from the block.
The bandwidth of a dielectric filter can be adjusted by changing the physical width of the dielectric block. Fine adjustment of the bandwidth typically requires the dielectric body to be machined to some degree to set it at the optimal bandwidth. These filters are usually made of ceramic material formed in a mold. Since it is not practical to make blocks of different width in the same mold, changing the frequency the filter is designed for can be difficult and expensive.
It is known that coupling between the resonators also controls the bandwidth. U.S. Pat. No. 4,255,729 of Fukasawa et al. discloses a series of individual resonators coupled together to form a filter. The coupling into the endmost resonators and between resonators is achieved either by current carrying loops of wire near the short circuit end of each resonator, which produce inductive coupling, or by conductive plates positioned near the open circuit ends of each resonator, which produce capacitive coupling.
The above-identified Sokola et at., Moutrie et al. and Green et al. patents illustrate that magnetic coupling between resonators in a single dielectric block can be controlled by unplated or plated holes through the block at locations between the resonators, and by grooves or slots on the surface of the body. Inductive coupling is also controlled by varying the dimensions of the dielectric body (e.g. by machining it) and varying the distance between resonators during manufacture. Capacitive coupling can be controlled by electrode patterns on the top or open circuit surface of the block.
In addition to adjusting the inter-resonator coupling in order to control the filter characteristics, it may also be necessary to adjust the center frequency of the filter. The center frequency can be adjusted by changing the length of the resonators, i.e. the distance between the top and bottom surfaces when the resonator holes extend from one surface to the other. The relationship is as follows: ##EQU1## where f.sub.c is the frequency in megahertz, l is the length and e.sub.r is the relative dielectric constant of the dielectric material. Since the body of dielectric material is typically a ceramic that is compressed in a mold, the height of a block can be varied without changing molds by controlling the amount of material placed in the mold and by making sure the open side of the mold corresponds to the top or bottom surface of the block.
Another way of controlling the center frequency is by adding capacitance to the open circuit end of the resonators. See, Matthaei et al., Microwave Filters, Impedance-Matching Networks, and Coupling Structures, McGraw-Hill, pp. 497-506 (1964). In effect, this capacitance foreshortens the resonator in that it lowers the resonant or center frequency. This allows the length of the resonator for the desired frequency to shorter than that specified by the equation given above. This capacitance can be achieved by means of plates positioned above the open circuit ends of the resonators as shown in U.S. Pat. No. 4,028,652 of Wakino et al.
The capacitance can also be achieved by an electrode pattern on the open circuit surface of the dielectric block as shown in the Sokola patent. After the dielectric filter is formed the frequency can be adjusted by removing conductor material near the open circuit end to raise the frequency and at the short circuit end to lower the frequency. This is described in U.S. Pat. No. 4,800,348 of Rosar.
With the prior art techniques the coupling into and out of the filter structure, as well as between resonators in a single dielectric block, is generally either capacitive or inductive. Also, when this coupling is accomplished by electrode patterns on the dielectric block, the patterns are typically on the open circuit side. Because of the holes which open onto this side, the arrangement of patterns is limited. Further, electrode patterns on the open circuit side cannot create inductive coupling.